Nearly 4 million Americans are currently infected with hepatitis C virus (HCV). HCV infection is responsible for approximately 8,000-10,000 deaths annually and is the leading reason for liver transplantation in the United States. There is no vaccine for the control of hepatitis C. A small number of people infected with.HCV overcome the virus during the initial infection and clear it from the bloodstream. The remaining 75%-85% develop a chronic infection. The severity of the disease varies greatly from person to person. The molecular basis for viral persistence, disease progression and potential virus/host targets for novel therapeutics is not well understood. We hypothesize that parenchymal HCV antigen presentation leading to inappropriate T cell function contributes to immune-mediated liver inflammation and that direct immunosuppression by viral proteins is a component of an integrated strategy of HCV survival. Three complementary approaches will be used to test these hypotheses: 1) To test the function of immune costimulatory molecules (e.g. CD40) in HCV-related liver injuries, we will establish a lineage of mice that conditionally express HCV structural proteins and CD40 molecules in the liver. Pathological changes in the liver, hepatic T cell priming and effector functions in these mice will be measured by FACS, ELISPOT and immunohistological analyses. 2) To examine whether HCV nonstructural (NS) proteins, especially NS3, interfere with the host antiviral responses leading to viral persistence, we will generate mice conditionally expressing HCV-NS genes, which are controlled by a novel tamoxifen-inducible system. We will examine these animals' interferon (IFN)-alpha/beta and adaptive immune responses in response to a viral challenge in the liver. Hepatic injury and viral persistence in these animals will be determined by biochemical, histopathological and immunochemical, and real-time PCR analyses. 3) Finally, we will examine the synergistic effects of HCV-S and-NS genes on disease pathogenesis and viral persistence in animals conditionally expressing the entire HCV genes in the liver. The immunosuppressive effect of the total complement HCV proteins will be tested following a viral challenge. These studies would provide important information concerning molecular mechanisms responsible for viral persistence and disease progression in vivo, and may aid in devising future strategies targeted at blocking specific candidates.